Newcomb's problem from the grand-system and petty-system views

In his original paper on what we now call the "many-worlds" interpretation, Everett motivated it with quantum cosmology, since there's nowhere outside the universe for a Copenhagen-style observer to stand. Eliezer Yudkowsky said something similar to motivate timeless decision theory: I hold it a virtue of any decision theory that it should be compatible with a grand-system view, rather than intrinsically separating the universe into agent and outside. All else being equal, I prefer a representation which is continuous over the grand universe and marks no special boundary where the observer is located; as opposed to a representation which solidifies the Cartesian boundary between an observer-decider homunculus and the environment. I think I can explain how it can be that a theory can require a Cartesian boundary but nobody seems to care or even really to notice, based on my experience in the more applied side of science. But I actually like the "petty-system" perspective of applications, and at the end I'll talk about how Newcomb's problem (or less ambiguous Newcomb-like problems) forces the issue of the "observer-decider" even without a grand-system view. The petty-system perspective in quantum mechanics It's pretty easy to do quantum mechanics every day and never think about interpretations. For example, using quantum chemistry software. I input a molecule as a file with a row for each atom, each row containing the atom's element identity and xyz coordinates. I include just the atoms in the molecule and not the atoms in its surroundings, as if the molecule is floating in outer space, which is enough for gas phase properties. You can do some impressive calculations with modern software. Erwin Schrödinger could calculate the hydrogen atom spectrum, but with a computer you can compute which frequencies of light will be absorbed by organic pigments such as those used to dye clothes or as food colorings. Of course for a dye you don't want the color of a gas but of a sol